Conventionally, as a method for purifying nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine via an exhaust pipe, an art is known in which urea water as a reducing agent is injected into the exhaust pipe and the nitrogen oxides are reduced and purified by a reduction catalyst arranged at the downstream side (Selective Catalytic Reduction).
For increasing purification efficiency of exhaust gas by this method, it is necessary to evaporate efficiency the urea water injected into the exhaust pipe and generate ammonia uniformly in the exhaust pipe.
For example, when the temperature of exhaust gas in the exhaust pipe is low, the urea water injected into the exhaust pipe cannot be evaporated efficiency. Accordingly, time is required for generating ammonia uniformly in the exhaust pipe, whereby the length of the exhaust pipe must be long.
However, since the length of the exhaust pipe is long in this construction, there is a problem in that large space is required for establishing the exhaust pipe. Furthermore, there is a problem in that the urea water tends to adhere to the wall surface inside the exhaust pipe and the adhering urea water is crystallized.
Then, an art is known in which a metal mesh member and a dispersing member mixing and dispersing urea water are arranged in the exhaust pipe oppositely to flow direction of exhaust gas (for example, see the Patent Literature 1).
According to this art, urea water injected into the exhaust pipe collides with the mesh member and fragmentized, and then mixed with the exhaust gas passing through the mesh member. The fragmentized urea water collides with the dispersing member provided downstream the mesh member so as to be mixed and dispersed efficiently in the exhaust pipe. Since the metal mesh member is heated by the heat of the exhaust gas, the evaporation of the urea water colliding with the mesh member is promoted.
Therefore, according to this art, ammonia can be generated uniformly in the exhaust pipe. Namely, it is not necessary to extend the length of the exhaust pipe, and large space for setting up the exhaust pipe is not required. Furthermore, the urea water is prevented from adhering to the wall surface inside the exhaust pipe and being crystallized.
However, according to the art described in the Patent Literature 1, the exhaust gas in the exhaust pipe passes through the mesh member and the dispersing member arranged oppositely to the flow direction of the exhaust gas. Accordingly, pressure loss of the exhaust gas occurs when the exhaust gas passes through the mesh member and the dispersing member. As a result, there is a problem in that pressure upstream the mesh member and the dispersing member in the exhaust pipe rises so that the performance of the internal combustion engine is decreased.
Therefore, according to the art described in the Patent Literature 2, a metal mesh member formed platy is established rotatably in the exhaust pipe. Then, when urea water is not injected, the mesh member is rotated so as to be directed in parallel to flow direction of exhaust gas. On the other hand, when the urea water is injected, the mesh member is rotated so as to be directed oppositely to the injection direction of the urea water (in other words, not parallel to the flow direction of the exhaust gas), whereby the urea water collides with the mesh member.
According to this art, when the urea water is not injected, the mesh member is rotated so as to be directed in parallel to the flow direction of the exhaust gas. Accordingly, pressure loss of the exhaust gas does not occur when the exhaust gas passes through the mesh member so that pressure upstream the mesh member in the exhaust pipe is prevented from rising, whereby the performance of the internal combustion engine is prevented from being decreased.    Patent Literature 1: the Japanese Patent Laid Open Gazette 2009-41371    Patent Literature 2: the Japanese Patent Laid Open Gazette 2010-38020